You do not need to use a commercial product <such as PartitionMagic> although feel free to do so if you are not comfortable using FDisk. Additionally, most flavors of Linux will come with a disk partitioning tool to help you with the installation.
I have had success using dual boot with Win98 and Linux as well as a triple with Win98, Linux, and Win2K. There are many, many sites that will out there that provide step by step help so just do a search for "dual boot".
HTH
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